This invention relates generally to the field of RF power generation, for applications including but not limited to particle accelerators. This invention relates particularly to the control of magnetron power sources for applications that require efficient regulation of output RF power and its phase over a broad range of amplitudes and phases. Usual magnetron power sources operate as oscillators, with constant output power provided by a regulated cathode voltage power source. To generate RF power matched to the frequency of an intended load, an injection phase-locking signal RF signal is fed into the magnetron through the magnetron output antenna. This is the usual operating mode for phase-locked magnetron RF power sources. However, to generate RF power matched to the power of a variable intended load, the usual technique is to divert some of the output power from the constant output power of the usual magnetron to a water-cooled dummy load.